Auto-ignited combustion in an HCCI engine depends strongly on the temperature, composition and pressure of the cylinder charge at intake valve closing. Hence, in order to achieve robust auto-ignited combustion, the inputs to the engine, such as the amount of fuel, fuel injection timing and intake/exhaust valve timings must be carefully coordinated to ensure that states of cylinder charge are within desired ranges.
Operating an HCCI engine using exhaust recompression strategy to control the cylinder charge temperature involves trapping the hot exhaust gas from the previous engine cycle by closing the exhaust valve early during the exhaust stroke and opening the intake valve at a late timing symmetrical to the exhaust valve closing timing. The cylinder charge composition and temperature will depend on how early the exhaust valve closes before the top dead center. If the exhaust valve closes earlier, a greater amount of hot exhaust gas from the previous engine cycle is trapped in the cylinder, leaving less cylinder volume for the fresh air mass, thereby increasing the cylinder temperature and decreasing the cylinder oxygen concentration. In the exhaust recompression strategy, the relationship between exhaust valve closing timing and intake valve opening timing is characterized by a “negative” valve overlap (as opposed to the typical positive valve overlap in a conventional internal combustion engine). The negative valve overlap (NVO) is defined as the duration in crank angle between exhaust valve closing and intake valve opening.
In addition to the valve strategy, a suitable fuel injection strategy must be used to achieve auto-ignited combustion for a wide-range of engine loads. For example, at a low engine load (for example, fueling rate <7 mg/cycle at 1000 rpm), the cylinder charge may not be hot enough for a stable auto-ignited combustion in spite of the highest practical value of NVO being used, leading to a partial-burn or misfire. One way to increase the charge temperature is to pre-inject a small amount of fuel near intake TDC (Top-Dead-Center) during the recompression. A part of the pre-injected fuel will reform due to the high pressure and temperature during the recompression, and the heat energy released from fuel-reforming will help increase the cylinder charge temperature enough for a successful auto-ignited combustion following the main fuel injection event. The amount of pre-injected fuel that reforms during the recompression depends on many variables such as injected mass, injection timing and trapped exhaust gas temperature and pressure.
It is desirable to precisely estimate and control the amount of fuel reforming because excessive fuel reforming decreases the fuel economy, while lack of fuel reforming may result in combustion instability.
However, it remains a significant challenge to estimate the amount of fuel that reforms during the recompression since fuel reformation depends on many variables such as injected mass, injection timing, and trapped exhaust gas temperature and pressure. In a HCCI engine, although one may use a cylinder pressure sensor to measure the combustion phasing and qualitatively relate it to the amount of fuel reforming, it is very difficult to isolate the effect of fuel reforming on combustion phasing from other engine variables. Furthermore, in-cylinder pressure sensing technologies are costly.
Therefore, what is needed is a robust and cost effective technique to determine the amount of fuel reformation in a HCCI engine employing exhaust gas recompression and fuel injection during exhaust gas recompression.